Spinal muscular atrophy (SMA) is a group of hereditary diseases that causes muscle damage leading to impaired muscle function, difficulty breathing, frequent respiratory infection, and eventually death. There are four types of SMA that are classified based on the onset and severity of the disease. SMA type I is the most severe form and is one of the most common causes of infant mortality, with symptoms of muscle weakness and difficulty breathing occurring at birth. SMA type II occurs later, with muscle weakness and other symptoms developing from ages 6 month to 2 years. Symptoms appear in SMA type III during childhood and in SMA type IV, the mildest form, during adulthood. All four types of SMA have been found to be associated with mutations in the SMN gene family, particularly SMN1.
Survival of motor neuron (SMN) is a protein involved in transcriptional splicing through its involvement in assembly of small nuclear ribonucleoproteins (snRNPs). snRNPs are protein-RNA complexes that bind with pre-mRNA to form a spliceosome, which then splices the pre-mRNA, most often resulting in removal of introns. Three genes encode SMN or a variant of SMN, including SMN1 (survival of motor neuron 1, telomeric), SMN2 (survival of motor neuron 2, centromeric), and SMNP (survival of motor neuron 1, telomeric pseudogene). SMN1 and SMN2 are a result of a gene duplication at 5q13 in humans. A lack of SMN activity results in widespread splicing defects, especially in spinal motor neurons, and degeneration of the spinal cord lower motor neurons.